Such charging devices are known to those skilled in the art. As illustrated in FIGS. 1 and 2, a charging device 20 usually comprises one or more primary antennas, called emitting antennas A1, A2, A3 situated beneath a charging surface Sc of the charging device 20. These emitting antennas A1, A2, A3 are connected to an electronic circuit (not shown in FIG. 1) which makes it possible to control the emission of each emitting antenna A1, A2, A3. A portable element 10, of the cell phone or other type, is placed on the charging surface Sc of the charging device 20 (cf. FIG. 2). This portable element 10 comprises a secondary antenna, called the receiving antenna Ar.
The operation of such a charging device is explained below. The electronic circuit detects the position of the receiving antenna Ar on the charging surface Sc, and instructs the emission of the emitting antenna(s) A1, A2, A3 of the charging device 20 which are the most aligned with the receiving antenna Ar. The detection of the position of the receiving antenna Ar is for example carried out by previously measuring a variation of voltage at the terminals of each emitting antenna A1, A2, A3. The emitting antenna(s) A1, A2, A3 which has (have) the greatest voltage variation is (are) substantially aligned with the receiving antenna Ar. One (or more) emitting antenna(s) A1, A2, A3 then emits (emit) a magnetic field B perpendicular to the current that passes through it (them), that is to say in the direction of the receiving antenna Ar. The magnetic field B is emitted at a determined frequency f which is the reception frequency of the receiving antenna Ar. The dimensions and structure of such charging devices 20 may be set by standards of the WPC (Wireless Power Consortium) type. According to this standard, for example the emission frequency of the emitting antennas A1, A2, A3 is between 100 kHz and 200 kHz.
In order to direct the magnetic field B preferably on the side on which the portable element 10 is situated, it is known practice to equip the charging device 20 with a resonant layer made of ferromagnetic material 30, also called ferrite 30, situated beneath the emitting antennas A1, A2, A3, that is to say on the side opposite to that on which the portable element 10 is situated. The magnetic field B is reflected by this ferrite 30 and is redirected mainly toward the portable element 10. The receiving antenna Ar receives this magnetic field B and the portable element 10 then converts the intensity of the magnetic field received by the receiving antenna Ar into a charging current. Charging stops when the portable element 10 sends an end-of-charging message to the charging device 20 in the form of a modulation of magnetic field which is received via the emitting antennas A1, A2, A3 and decoded by the electronic circuit.
In order to obtain a magnetic field B that is as uniform as possible on the charging surface Sc, it is known practice to place in the charging device 20 at least two superposed layers of emitting antennas A1, A2, A3 parallel to the charging surface Sc, the antennas situated on a top layer being offset relative to the antennas situated on a lower layer. As illustrated in FIG. 2, the emitting antenna A2 is situated above the emitting antennas A1 and A3 and it overlaps a portion of each of these emitting antennas A1, A3. However, the magnetic field B that is emitted (cf. FIG. 3) is not completely uniform over the charging surface Sc of the charging device 20 (cf. FIG. 2). The portable element 10 receives a magnetic field of different intensity depending on its position on the charging surface Sc. This is illustrated in FIG. 3 where the intensity of the magnetic field B is shown on a longitudinal axis X of the charging device 20. It emerges clearly from this FIG. 3 that the intensity of the magnetic field B is substantially uniform over a central portion of the charging surface Sc, between the positions x1 and x2, but that at the edges of the charging device 20, from the position 0 to the position x1 and from the position x2 to the position L, the intensity of the magnetic field B drops considerably. This is partly due to the dissipations of the magnetic field B at the edges of the charging device 20. Since the intensity of the magnetic field B is insufficient at these ends, charging of the portable element 10 is either impossible there or takes an abnormally long time.
This charging device 20 is known to those skilled in the art. The electronic circuit 40 incorporated into the charging device 20 which controls the emission of the magnetic field B is illustrated in FIG. 4. It comprises a control system S, of the microprocessor type, connected to a transmission unit T and to an array of three input switches Se1, Se2, Se3 (for example mechanical relays or transistors) each connected to an emitting antenna A1, A2, A3. Each emitting antenna A1, A2, A3 is also connected to an output switch Ss1, Ss2, Ss3 and then to at least one impedance-matching capacitor Ca and finally to a reception unit R, itself connected to the control system S.
The input switches Se1, Se2, Se3 are used to select the emitting antenna A1, A2, A3 emitting the magnetic field B in order to charge the portable element 10. The output switches Ss1, Ss2, Ss3 for their part are used to select an emitting antenna A1, A2, A3 to receive the messages originating from the portable element 10 such as an instantaneous charging rate or an end-of-charging message. Usually, it is the same emitting antenna A1, A2, A3 that receives the messages from the portable element 10 and that is selected to charge the latter. This emitting antenna A1, A2, A3, once selected, is then connected to an impedance-matching capacitor Ca, making it possible to adapt its emission frequency and hence that of the magnetic field B to that desired for the charge. For example, according to the example illustrated in FIG. 2, the emitting antenna A2 of the charging device 20 being ideally aligned with the receiving antenna Ar of the portable element 10, this emitting antenna A2 is selected for the transmission of the charge (emission of the magnetic field B) to the portable element 10 and for the reception of the messages originating from the latter. However, it is more frequent that no emitting antenna A1, A2, A3 of the charging device 20 is directly and ideally aligned with respect to the receiving antenna Ar of the portable element 10 and that it is necessary to select two adjacent emitting antennas A1, A2, A3 in order to charge the portable element 10 in an optimal manner (the shortest possible charging time).
A first drawback of this charging device 20 then appears. The use of two adjacent emitting antennas A1, A2, A3 to charge the portable element 10 creates a magnetic field B covering a charging surface area markedly greater than that theoretically necessary to charge the receiving antenna Ar. There is therefore a portion of magnetic field B emitted that is dissipated, unusable for the portable element 10 and hence wasted. This dissipation is added to that present at the edges [0, x1], [x2, L] of the charging device 20 (as explained above) thus creating an overconsumption of energy.
A second drawback of this charging device 20 lies in the use of an input switch Se1, Se2, Se3 and of an output switch Ss1, Ss2, Ss3 for each emitting antenna A1, A2, A3. These switches are traversed by high charging currents (>1 A) and are therefore relatively costly because they are adapted to support these high currents.
A third drawback lies in the use of several layers of emitting antennas A1, A2, A3 that are offset relative to one another, which adds a considerable extra cost to the charging device 20, each emitting antenna A1, A2, A3 being accompanied by its input switch, output switch, etc.
It is these drawbacks that the present invention proposes to alleviate.